Inorganic substances such as selenium, cadmium sulfide, zinc oxide and amorphous silicon are well known as photoconductive compositions which can be used in electrophotographic photoreceptors.
These inorganic photoreceptors are distinguished by having good electrophotographic properties, which is to say by providing very good photoconductivity and charge accepting properties, and insulating properties in the dark. However, they also have various disadvantages. For example, selenium photoreceptors are expensive to manufacture, they lack flexibility and are easily damaged by heat or mechanical shock. Cadmium sulfide photoreceptors give rise to problems with pollution because of the toxic material cadmium which is used in them. Zinc oxide has the disadvantage of being unable to provide image stability when used repeatedly over long periods of time. Moreover amorphous silicon photoreceptors are very expensive to manufacture and require special surface treatments to prevent deterioration of the photoreceptor surface.
Electrophotographic photoreceptors in which various organic substances are used have been proposed in recent years, and these have been used in practice. For example, there are electrophotographic photoreceptors comprised of poly-N-vinyl-carbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237), electrophotographic photoreceptors in which poly-N-vinylcarbazole is sensitized with pyrylium based dyes (JP-B-48-25658), and electrophotographic photoreceptors in which co-crystalline complexes comprised of dyes and resins form the principal components (JP-A-47-10735). (The terms "JP-A" and "JP-B" as used herein signify an "unexamined published Japanese patent application" and an "examined Japanese patent publication" respectively.)
Furthermore, electrophotographic photoreceptors in which organic pigments such as perylene pigments (for example, U.S. Pat. No. 3,371,884), phthalocyanine pigments (for example, U.S. Pat. Nos. 3,397,086, 4,666,802), azulenium salt based pigments (for example, JP-A-59-53850, JP-A-61-212542), squalium salt based pigments (for example, U.S. Pat. Nos. 4,396,610, 4,644,082) and polycyclic quinone based pigments (for example, JP-A-59-184348, JP-A-62-28738), and azo pigments such as those indicated below form the principal component have been actively researched, and a great many such materials have been suggested.